The invention pertains to microwave test instrumentation and in particular to an instrument employing a delay line and a phase detector capable of sub-microsecond modulation domain testing.
Electronic simulators are capable of replicating high density radar signals. One such simulator employs computer modeling to formulate real time parameters of platforms and emitters in the environment, as well as microwave hardware to generate the actual microwave signals received by the system under test. The modeling entails calculating, in real time, parameters such as platform position, propagation path loss, and emitter events, while the microwave hardware generates pulses with the appropriate parameters.
The design of generators useful in such simulators is dependent on the speed of the settling time of microwave voltage controlled oscillators (VCOs) employed in the design. In order to provide for interleaved radar pulses, for example, the generator must switch from any frequency to any other frequency in the desired range, e.g. 500 MHz to 18 GHz to any other frequency in the range in under 300 ns. For an average pulse width of one microsecond, the VCO settling time significantly affects the pulse through put capability of the system. Further, if the VCO frequency settling time is slower than required, unintentional frequency-modulation-on-pulse (FMOP) will result. This unintentional FMOP is unacceptable. The simulator is designed to accurately simulate fine grain FMOP, which it cannot do if there is residual FMOP due to the VCO settling characteristics.
There are currently no instruments commercially available which can measure the frequency settling time with the required resolution. This measurement is in the modulation domain, that is, frequency versus time. Neither a spectrum analyzer nor an oscilloscope is capable of making modulation domain measurements. The spectrum analyzer measures amplitude versus frequency but cannot acquire the time information. The oscilloscope measures amplitude versus time, but cannot acquire the frequency information.
Modulation domain analyzers, such as HP-5371A and HP-5372A manufactured by Hewlett-Packard, use a gated frequency counter technique and lack the necessary accuracy of resolution. These instruments use digital frequency counter techniques, and are thus limited in accuracy and resolution by the sampling rate. For example, the sampling rate of these instruments is not sufficiently fast to give a frequency accuracy of 0.1 MHz, and a time resolution of 12.4 ns, as required for high speed VCO settling measurements.
Therefore, there is a need to accurately measure the modulation domain performance of the voltage controlled microwave oscillators within the system.